Power supplies for computers, personal digital assistants, cellular phones and other hand held mobile electronic devices and systems have exacting demands. A buck converter functions to step down a high voltage to a lower voltage so that it is compatible with, for example, a CPU on a motherboard for a personal computer. Typically, the buck converter operates using a clock, whereby an inductor is charged or energized during a first portion of a clock cycle (“charging phase”) and operates as a current source during the second portion of the clock cycle (“discharging phase”).
A typical buck converter having a synchronous rectifier topology operates by commencing the charging phase in response to a clock signal. During the charging phase, the inductor, capacitor and load are coupled to the input voltage. Meanwhile, the inductor current is monitored, and, when it peaks at a predetermined value, the converter is decoupled from the input voltage and the inductor discharges its energy through the load. Because the separation between the charging and discharging phases is defined at the point in time at which the inductor current peaks at the predetermined value, this type of buck converter is commonly referred to as a “peak current control” buck converter. The inductor current rises and falls linearly according to is the voltage across the inductor.
A buck converter can also be characterized as a step-down switch-mode power supply where the average output voltage can be shown to be directly proportional to the converter duty cycle, D which is the portion of the buck converter clock period during which high-side switch is on.
Buck converters are characterized by having a high side transistor and a low side transistor. Today most buck converters are made with mosfets for the high and low side transistors. Power mosfets are well documented and are often used in buck converters to perform DC to DC conversion.
However, as systems such as handheld devices and cell phones continue to shrink in size, it becomes more important reduce the area on a system circuit board for any given system function. Where two mosfets are wired together on a system board, the area assigned to the wired mosfets is generally more than the area of the individual mosfets because the mosfets in a buck converter must be connected together. Conventional assembly techniques dispose mosfets laterally on a system board. Such assemblies increase parasitic effects due to wiring. As such, it would be beneficial to reduce parasitic performance, reduce the area allotted to the buck converter mosfets and improve the speed and reduce the complexity of assembling a buck converter on a system board.